The present invention relates to a pressure transducer wherein through holes are bored in a detection block portion such that easily-deformable parallel thin portions are formed between the upper and lower hole walls of the through holes and the upper and lower surfaces of the detection block portion.
When a tunnel is excavated in soft ground by a shield excavator while the amounts of excavated earth and exhausted earth are balanced, earth excavated by the rotary cutter of the excavator is packed into a chamber separated by bulkheads, and it is detected whether or not the earth pressure in the chamber is exactly equal to the static earth pressure at a facing which keeps the facing stable. An earth pressure cell employed for this purpose must be able to deliver a correct mean stress constantly even when it receives a concentrated eccentric load owing to the presence of pebbles and the like. A similar ability is also needed, and actually tried to be sought, in various fields of technology, such as when an investigation is made of the forces on the inside of a silo for the purpose of designing and controlling the silo.
The inventor of the present application has suggested several types of load converters which can meet the aforementioned requirements, some of which will be described hereinafter.
One is the constitution wherein parallel flat plates are formed by boring a through hole having a square sectional shape in a detection block. This structure is shown in Japanese Patent Application published under No. 124169/1979.
Another is the improved constitution wherein a through hole is provided by cutting off the section between two small round holes and connecting them instead of having a square hole, in view of such inconvenience as in preparation of the parallel flat-plates by a through hole of square section. This is not only very easy to prepare but also capable of fulfilling a function equivalent to the function of the aforesaid parallel flat plates. This is disclosed in my co-pending U.S. patent application Ser. No. 433,109 filed Oct. 6, 1982.
In order to facilitate the understanding of the present invention, an outline of the invention disclosed in the aforementioned co-pending U.S. application is explained with reference to FIG. 1.
In FIG. 1, a detection block 1b is formed in the shape of a chest with rounded in surfaces as illustrated, and small round holes 3 are formed at upper and lower positions in the direction perpendicular to the longitudinal direction of the block. Extended through holes 2 are formed with their upper ends having a larger distance between them than that between the lower ends thereof, by connecting two round holes 3 together through a connection passage 4. A pressure receiving portion 5 located at the middle of the longitudinal detection block 1b, which is held between the two through holes 2 thus prepared, and retaining portions 6 located on both sides of the pressure receiving portion 5 are connected together by upper and lower parallel thin wall portions 7 formed between each of the upper and lower round holes 3 and the upper and lower surface of the detection block 1b against which the upper and lower round hole portions 3 confront. A pressure receiving plate 1a is formed above the pressure receiving portion 5 in a unitary structure, while a circular base 1c supporting the retaining portions 6 in a built-in beam manner is formed below the portions 6, and an opening 8 is formed in the base 1c. When the pressure receiving plate 1a is loaded in the axial direction, the pressure receiving portion 5 in the middle of the detection block 1b is displaced in the vertical direction so that its center of gravity moves parallel. Therefore, the displacement and the force applied can be determined by a displacement detection device or a force detection device provided underneath, and in addition, since compression and tension strains are generated in the parallel thin portions 7 which are each bent, strain outputs can be obtained by attaching four strain gages to the portions 7 and forming them into a Wheatstone bridge.
However, since the parallel thin portions located at four upper and lower portions are configured so that they extend in the same, single direction, the characteristics thereof and the results obtained are influenced by the direction of the load and differ in accordance with whether the direction of the load is the same as that direction or is perpendicular thereto.
Although it may be possible to improve the transducer by changing the linear format of the detection block 1b to cross-shaped format, such a change in format configuration still provides an opening 8 formed in either case between the detection block 1b and the base member 1c, and this brings about another disadvantage that the detection of the load transduced into liquid pressure of a liquid enclosed inside the circular base member 1c is impossible and the formation of an explosion-proof terminal box wherein an inactive gas is enclosed is also impossible with the structure of the known transducer.